Function render_diagnostic 

pub fn render_diagnostic(
    diag: &ResolvedDiagnostic,
    opts: &OutputOptions,
) -> String

Render a ResolvedDiagnostic to a String. The returned string ends with a newline. Produces:

• file:line:col: kind: message\n
• the source line + \n (if available)
• the caret/underline line + \n (only for all-ASCII source lines)

Port of printDiagnosticHelper.